Voltage control for a motor supply system

ABSTRACT

In this system, a controlled rectifier converter interconnects an alternating current voltage source with an AC induction motor. To effect voltage amplitude control, trigger signals are pulsed to the various controlled rectifiers of the converter during their respective 120* conductive intervals. The resultant duty cycle modulation affords control of the level of the voltage applied to the induction motor. At low frequencies, a plurality of power pulses are provided by the converter during each 60* of conduction, whereas at high frequencies, a single power pulse is supplied for each 60* of conduction. A smooth transition is provided between these two operating modes to ensure smooth, continuous motor control and operation throughout the operating range of the motor. This smooth transition is facilitated by synchronizing the power pulses in relation to the two 60* increments comprising the 120* conductive intervals for the respective controlled rectifiers.

United States Patent Salihiet al.

[541 VOLTAGE CONTROL FOR A MOTOR SUPPLY SYSTEM [72] Inventors: Jalal T.Salihi, Birmingham; Daniel W.

Shimer, Madison Heights, both of Mich.

[73] Assignee: General Motors Corporation, Detroit,

Mich.

221 Filed: Feb. 22, 1971 211 Appl.No.: 117,498

[ 51 3,659,168 51 Apr. 25, 1972 Primary Examiner-Gene Z. RubinsonAttorney-E. W. Christen and C. R. Meland [57] ABSTRACT In this system, acontrolled rectifier converter interconnects an alternating currentvoltage source with an AC induction motor. To effect voltage amplitudecontrol, trigger signals are pulsed to the various controlled rectifiersof the converter during their respective 120 conductive intervals. Theresultant duty cycle modulation affords control of the level of thevoltage applied to the induction motor. At low frequencies, a pluralityof power pulses are provided by the converter during each 60 ofconduction, whereas at high frequencies, a single power pulse issupplied for each 60 of conduction. A smooth transition is providedbetween these two operating modes to ensure smooth, continuous motorcontrol and operation throughout the operating range of the motor. Thissmooth transition is facilitated by synchronizing the power pulses inrelation to the two 60 increments comprising the 120 conductiveintervals for the respective controlled rectifiers.

9 Claims, 7 Drawing Figures A S3 G Hill? GATEFIRING sup FREQUENCYCONTROL 32 Patented April 25, 1972 3 Sheats-Sheet l QIW N OmWm Ow mmmmwmQ ZOE .rUm E m OVN Om:

INVIjN'lY/RS Ja/a/ f 50/2/21 8 BY $02226 M S/zimer W ATTORNEY PatentedApril 25, 1972 3,659,168

3 Sheets-Sheet 2 SAWT OOTH ADDER GENERATOR 24 VOLTAGE COMPARATOR DIRECTVOLTAGE LOW PASS TRIGGER CIRCU ITRY 1 06074) I72 JU'II'U'U'IFUU U U U UUU U U U U UU U A+ nmmnmnnnnnnn TRIGGER \VDC (174') LFL m L A+ TRIGGERKW/ M ATTORNEY Patented April 25, 1972 3 Sheets-Sheet 3 -w i. .m M w.with z j I 98 5/ 1Z4" 126 MONOSTABLE 70 10 MULTIVIBRATOR 3 VO LTAG E COM PA RATO F? AT TOR N E Y VOLTAGE CONTROL FOR A MOTOR SUPPLY SYSTEM Thisinvention relates to a converter for providing an AC output havingcontrollable frequency from a DC or fixed frequency AC input whereinduty cycle modulation of converter controlled switches provides controlof the output voltage level.

Converter arrangements are generally known in the art of motor speedcontrol for supplying variable frequency power to drive AC inductionmotors. For example, inverters are generally known for motor controlapplications using DC power sources, and cycloconverters are known foruse in motor control circuits to provide a controlled, variablefrequency AC motor drive voltage from a fixed frequency AC source. Inthese known systems, the frequency of trigger signals gating theconverter controlled switching devices conductive is controlled in apredetermined manner to effect the requisite frequency control. Thecontrolled switches can take the form of controlled rectifiers,transistors or other controllable switching devices according to therequirements of the particular application. Voltage amplitude regulationis accomplished in these known systems by controlling the voltage levelof the AC or DC power source input to the converter. Accordingly,control of the total operation of the motor is accomplished by controlof the switching frequency and control of the input voltage.

The instant invention, by means of duty cycle modulation, affordsvoltage control without the necessity of controlling the input voltageto the inverter or cycloconverter. To accomplish the requisite dutycycle modulation, a sawtooth waveform is generated having a frequencyrelated to the switching frequency of the converter. At low operatingfrequencies, this sawtooth waveform completes several cyclesintermediate each pair of trigger signals, whereas at high frequencies,a single cycle of the sawtooth waveform is completed for each pair oftrigger signals. The sawtooth waveform has a constant maximum amplitudeindependent of the instantaneous frequency of the sawtooth signal. Ascheme of synchronization of the sawtooth voltage in relation to thetrigger signals ensures a smooth transition between the low frequencyand high frequency modes of operation.

Voltage control of the converter output is accomplished by comparing areference DC voltage level with the sawtooth voltage waveform. In thismanner, the system of the present invention develops a control signal toregulate conduction by converter controlled rectifiers during theintervals in which they otherwise would be provided gate trigger pulses.The normal trigger pulses to the controlled rectifiers are thusinhibited by inhibiting the operation of the trigger source when thereference DC and the sawtooth voltage have a first predeterminedamplitude relationship; additionally, the trigger pulses are applied tothe respective controlled rectifiers when the two voltages have a secondpredetermined amplitude relationship by enabling the trigger source atthat time. For example, the controlled rectifier trigger source can beinhibited when the instantaneous amplitude of the sawtooth voltage isless than the reference DC signal and enabled when the instantaneousamplitude of the sawtooth waveform exceeds the DC reference.Accordingly, modulated trigger pulses are supplied the controlledrectifiers during their respective conductive intervals.

As is known in the art of motor control, during low frequency operationit is desirable to provide a plurality of power pulses to supply a motorfrom a converter rather than a single pulse followed by an extendedperiod of nonconduction. Two considerations underlie the choice todistribute the single power pulse in the form of a plurality of shorterpulses: first, the resultant voltage has a lesser harmonic content andis, accordingly, a more efficient mode of operation; second, mechanicaleffects of torque pulsations are reduced and smoother operation obtains.The present invention contemplates optimum low speed operationconsistent with the above criterion and high speed operation with asingle power pulse intermediate each pair of gating signals withprovision for a smooth transition between the modes.

It is an object of the present invention to provide a converter motorsupply wherein voltage magnitude control is effected by duty cyclecontrol of the conductive intervals of controlled switching devices inthe converter.

It is another object of the present invention to provide a controlledrectifier converter interconnecting an alternating current source withan AC induction motor capable of controlled frequency operation whereintrigger pulses to the converter controlled rectifiers are periodicallyinterrupted to regulate the magnitude of voltage supplied to the ACmotor. Another object of the present invention is to provide acontrolled rectifier converter of the type described wherein duty cyclecontrol is used to regulate voltage output and wherein a low frequencymode including a plurality of control pulses intermediate each pair oftrigger signals and a high frequency mode having a single control pulseare provided.

Another object of the present invention is to provide a converter supplysystem for energizing an AC motor having low frequency and highfrequency operating modes of the type described wherein a smoothtransition between the modes of operation is accomplished bysynchronizing the trigger pulses applied to the converter relative tothe trigger signals defining 60 degree increments of converter operationin a manner permitting a continuous transition from the initiation ofmotor operation to the frequency preselected as defining the beginningof high frequency operation.

These and additional objects and advantages of the present inventionwill be apparent from the following description wherein the FIGS. listedbelow are incorporated as illustrating a preferred embodiment.

In the drawings:

FIG. 1 is a schematic circuit diagram of a power supply system for aninduction motor including a controlled rectifier converter.

FIG. 2 is a timing chart graphically depicting the voltages applied theinduction motor phase windings of FIG. 1 during intervals of fullvoltage operation.

FIG. 3 is a block diagram of gate firing trigger circuitry made inaccordance with the present invention to develop the requisite enablingand disabling control to effect duty cycle modulation of the converterof FIG. 1.

FIGS. 4A, 4B, and 4C are timing diagrams depicting the operation of theconverter of FIG. I as controlled by the angle control or duty cyclemodulation circuitry of FIG. 3.

FIG. 5 is a detailed schematic of the oscillator and sawtooth generatorcontrol and synchronization scheme of the present invention.

Reference should now be made to the drawings and more particularly toFIG. 1 wherein an electric propulsion system for a vehicle isillustrated including a source of polyphase alternating currentdesignated by reference numeral 10. This source of alternating currentis a three-phase Y-connected generator having an output winding 12 and afield winding 14. Field winding 14 is serially connected with a sourceof direct current 16 through a variable resistor 18. Variable resistor18 provides regulation of field current and concomitantly the generatoroutput voltage. It should be appreciated that variable resistor 18 ismerely illustrative of various field current control devices: inpractice, it could take a variety of forms, for example, a transistorvoltage regulator might be used.

One use for the system to be described is the electric propulsion of avehicle such as an earth mover in which case field winding 14 would becarried by the generator rotor (not illustrated) and driven by a primemover such as a turbine or diesel (not illustrated). It should beappreciated that the source of alternating current could be three-phasecommercial power if the present invention were incorporated in a motorcontrol system in a manufacturing facility.

Three power supply conductors 20, 22, and 24 provide an interconnectionbetween the Y-connected winding 12 and the controlled rectifiers of theconverter. These three conductors carry three-phase voltage having afrequency and amplitude determined by the operating conditions of thealternating current source 10. For purposes of the following discussion,the

frequency and magnitude of the voltage on conductors 20, 22, and 24 areassumed to be constant.

In the earthmover application, the three-phase induction motor 28 havingphase windings A, B, and C provides drive power for vehicle propulsion.In this situation, the squirrel cage rotor 30 is coupled with a vehiclewheel 32 to drive the vehicle. A mechanical differential or othercoupling can be used between the rotor 30 and the vehicle wheel 32.Depending on the particular operating characteristics which are desired,each wheel can be driven by a separate motor or one motor can drive twowheels through a differential.

Six full wave bridge rectifying networks control the frequency of powersupplied the phase windings A, B, and C from the source of alternatingcurrent 10. Three AC input terminals 34 are provided to a first fullwave bridge comprised of three controlled rectifiers 36A and threecontrolled rectifiers 38A. The input terminals 34 are connectedrespectively with the three conductors 20, 22, and 24 of the drawing.This bridge rectifier network has direct current output terminalsconnected with conductors 40 and 42. It will be appreciated by thoseskilled in the art that a direct voltage is developed across conductors40 and 42 when the six controlled rectifiers 36A and 38A aresimultaneously gated conductive. This voltage has a positive polarity onconductor 40 and a negative polarity on conductor 42. Phase winding A isconnected by conductors 44 and 46 with the direct current output of thefull wave bridge and, accordingly, current will flow through the phasewinding A from conductor 44 to conductor 46 when controlled rectifiers36A and 38A are conductive. The voltage developed across phase winding Aduring the interval in which controlled rectifiers 36A and 38A areconductive is depicted in FIG. 2 and labeled A+.

A second three-phase full wave bridge rectifier comprised of threecontrolled rectifiers 48A and three controlled rectifiers 50A providesdirect current of the opposite polarity of that described to phasewinding A of the induction motor 28. When the six controlled rectifiers48A and 50A are simultaneously gated conductive, conductor 42 provides apositive output with respect to conductor 40 such that a current flowsthrough phase winding A from conductor 46 to conductor 44. The resultantvoltage developed across phase winding A is depicted in FIG. 2 and thereidentified as A. It should be appreciated that the conduction times forthe two full wave bridge rectifiers described are nonoverlapping. Thefull wave bridge rectifiers carry additional indicia A+ and A- in thedrawing of FIG. 1 to correlate them with the voltages developed, duringtheir respective conductive intervals, across phase winding A anddepicted in FIG. 2.

Four additional full wave bridge rectifiers are included in the circuitschematic of FIG. 1 to energize motor phase windings B and C in a manneranalogous to that described for motor phase winding A. To supply phasewinding B with current of a polarity to develop voltage B+ of FIG. 2,six controlled rectifiers 52B and 54B are simultaneously gatedconductive. In a similar manner, voltage B- of FIG. 2 is developedacross phase winding B when the six controlled rectifiers 56B and 58Bare gated conductive. Controlled rectifiers 60C and 62C supply currentto phase winding C having a polarity such that voltage C+ of FIG. 2 isdeveloped across the phase winding. Finally, controlled rectifiers 64Cand 66C supply phase winding C with current to develop voltage C- ofFIG. 2 when these six controlled rectifiers are provided simultaneousgate signals. A

It is noted that the voltages developed across the various phasewindings as depicted in FIG. 2 have a theoretical waveform ignoringripple and delay on shut off. It should also be appreciated that theoperation as described assumes full application of the voltage availablefrom the source for the entire 120 conductive intervals of the variousfull wave bridge rectifiers. The modification of this conductionnecessary to accomplish voltage amplitude control according to thepresent invention is set forth hereinafter in conjunction with FIGS. 3,4A, 4B, and 4C.

Operation of the induction motor 28 obtains in accordance with theperiodic and sequential switching of the six full wave bridge networksshown in the drawing. The operation is summarized graphically for onecycle in FIG. 2. At any given time, two phase windings are energized asshown in the diagrams of FIG. 2. One of the two phase windings isprovided positive voltage, while the other is provided negative voltage.For example, from zero to 60, phase windings A and B are simultaneouslyenergized; A is energized with a positive voltage A+, and B is energizedwith a negative voltage B. During this 60 interval, the twelvecontrolled rectifiers 36A, 38A, 56B, and 58B are conductive, and theremaining 24 controlled rectifiers are nonconductive.

The 36 controlled rectifier converter generally described above is ofthe nonsynchronized type inasmuch as no attempt is made to correlate thegating of controlled rectifier with any of the voltages of the outputwinding 12. It should be appreciated that the controlled rectifiers aregated periodically and sequentially in groups of six and that the gatepulses are randomly applied when compared with the voltage waveforms ofthe voltages on conductors 20, 22, and 24. Each controlled rectifier ismaintained in the conductive state as long as a gate pulse is applied toit, and commutation is inherent after the gate pulse is removed when theAC voltage from the source 10 connected to the particular controlledrectifier falls below the level necessary to sustain conduction by thecontrolled rectifi- Frequency control of the voltage applied to theinduction motor 28 is effected by varying the switching frequency of thecontrolled rectifiers included in the various full wave bridges of thedrawing. Voltage amplitude control is obtained by duty cycle modulationof the controlled rectifiers during their respective conductiveintervals in a manner more fully discussed hereinafter. Thus, it shouldbe appreciated that the assumption noted above relating to the constantfrequency, constant amplitude character of the source 10 does not limitthe control of the power supplied the induction motor 28 since bothfrequency and voltage are controllable by proper control of theconverter controlled rectifiers.

The switching frequency of the controlled rectifiers can be regulated inaccordance with several known control arrangements. Suitable controlsystems providing slip frequency control for induction motors are shownin detail in the U.S. Pat. Nos. to Salihi et al. 3,471,764 and toAgarwal et al. 3,323,032. In the system of the drawing of FIG. 1, slipfrequency control is accomplished by a tachometer 68, an adder 70, and aslip frequency control 72. The tachometer 68 develops a voltage which isa function of the speed of the induction motor rotor 30. This tachometer68 can take a variety of known forms including a generator providing aDC output or a pulse tachometer capable of developing a series of pulseshaving a frequency which is a function of rotor speed. The adder 70 issupplied an input from both the tachometer 68 and the slip frequencycontrol 72.

In operation, the adder 70 provides an output signal correlated with thesum of the input signals from the tachometer 68 and the slip frequencycontrol 72. It should be appreciated that manual or automatic regulationof the signal from slip frequency control 72 affords control of the slipfrequency of the induction motor 28. Thus, the motors performancecharacteristics as affected by the instantaneous slip frequency arecontrollable.

The adder 70 provides a pulse output connected by conductor 74 with agate firing array 76. Trigger pulses for the various full wave bridgerectifiers are generated in response to the trigger signals from theadder 70. Specifically, each signal from the adder 70 causes a triggerpulse to be applied to the six controlled rectifiers of a particularfull wave bridge. Proper sequencing of the trigger pulses is provided bylogic in the gate firing array 76. Six output arrows are included in thedrawing to represent the trigger pulses developed by the gate firing 76.Each of the arrows is identified by notation indicating the bridgerectifier controlled thereby, and it should be appreciated that a gateand cathode connection is required between each of the 36 controlledrectifiers and the gate firing in the control circuit.

A system for developing gate firing signals of the type required iscompletely described in copending application, Ser. No. 57,143, filedJuly 22, 1970, in the name of Jalal T. Salihi et a1. and entitledControlled Rectifier Triggering System.

In the preferred embodiment, the system of copending application, Ser.No. 57,143 is used in conjunction with the voltage angle control orpulse modulation arrangement of the present invention. Accordingly, theoperation of the trigger system of Ser. No. 57,143 insofar as pertinentto the present invention is summarized below.

In the controlled rectifier triggering system of Ser. No. 57,143, alogic array (not illustrated) provides periodic and sequential enablingsignals to initiate group gating pulses for application to the sixcontrolled rectifiers of the full wave bridge networks. To initiate agate pulse for a particular group of controlled rectifiers comprising afull wave bridge, a high frequency chopper provides a pulsating input tothe primary winding of a transformer when a transistor switch is biasedconductive in response to a logic control signal. The transformer isprovided six secondary windings, one each for the six controlledrectifiers of the bridge group. Each secondary of the transformer isconnected with a rectifier network and the resultant DC gate pulse isapplied through a trigger circuit to the controlled rectifier connectedwith the particular secondary winding. Thus, it should be appreciatedthat the six controlled rectifiers of each full wave bridge rectifierare gated when the transistor control element associated with theprimary windings of the transformer connected with the controlledrectifiers of that full wave bridge is biased conductive. Inasmuch asthis summary is adequate for an understanding of the present invention,the remaining details of the controlled rectifier triggering system arenot set forth nor illustrated herein and reference should be made tocopending application, Ser. No. 57,143 for the details of the entirecircuit required.

For the following explanation, it is understood that the gate triggersignals available on the conductor 74 are spaced by 60 increments andprovide the requisite control to the gate firing array 76 connected withthe controlled rectifiers to produce the switching required to providethe voltage pattern of FIG. 2. Referring to FIG. 2, it should beappreciated that the trigger signals on conductor 74 occur at zero, 60,120, 180, 240, 300, 360, etc. During application of full voltage to thephase windings A, B, and C, the voltage pattern of FIG. 2 obtains. Thetrigger pulses or signals from gate firing 76 required for thecontrolled rectifiers to develop this voltage pattern are provided inthe manner set forth in the summary above. On the occurrence of eachtrigger signal on conductor 74, one control transistor is supplied abias voltage and concurrently a second transistor is renderednonconductive as the bias voltage connected with it is removed. Thecontrol transistor which is switched nonconductive will have beenconductive for 120. This pattern of control continues and the voltagesof the graph of FIG. 2 are developed and applied to the motor accordingto the bias sequence and timing for the six control transistors.

Reference should now be made to FIG. 3 wherein a block diagram of thegate firing array 76 of FIG. 1 is shown including the control necessaryto obtain voltage control by pulse modulation of the controlledrectifiers during their respective conductive intervals. As noted above,the adder 70 provides pulses spaced by 60. These pulses are applied tothe input of an oscillator 78 and directly to a pulse summer 80.Oscillator 78 provides a pulse output intermediate each pair of pulsesfrom the adder 70 at a constant, fixed frequency; the oscillator 78 isdisabled when the frequency of the pulses from the adder 70 on conductor74 exceeds the fixed frequency of the oscillator 78. The constantfrequency pulse output from oscillator'78 is connected with the summer80 on line 82 and the combined output online 84 from the summer 80 issupplied to a sawtooth generator 86. In the timing diagrams of FIGS. 4A,4B, and 4C, the pulses from the adder 70 are labeled 74, the pulses fromoscillator 78 are labeled 82, and the combined output from the summer 80is labeled 84. These indices in FIGS. 4A, 4B, and 4C are the same as thereference numerals of the conductors carrying the pulses in the drawingof FIG. 3. It should be appreciated that the pulse train 84 is thesummation of the pulses in the two trains 74 and 82. In FIG. 4C, thefrequency of the pulses of the pulse train 74 exceeds the constantfrequency of the oscillator 78, and accordingly, there are no pulsesshown for the output conductor 82. Thus, the pulse train 84 isidentically the same as the pulse train 74 in FIG. 4C.

Reference should now be made to FIG. 5 wherein a detailed circuitschematic of the oscillator 78 and the sawtooth generator 86 is shown.The following description of the operation of the oscillator andgenerator is directed to the circuitry shown; however, it should beappreciated that a variety of alternative circuit configurations wouldsuffice in the control arrangement.

In FIG. 5, the pulses from the adder 70 on conductor 74 are connectedthrough a resistor 88 with the base electrode of a transistor 90. Acapacitor 92 is connected across the emitter and collector electrodes ofthe transistor 90, and a second transistor 94 together with theresistors 96, 98, and comprises a constant current charging source forthe capacitor 92. A source of direct voltage shown as a battery 102provides voltage and current for the oscillator circuitry.

In operation, each pulse on conductor 74 coupled to the base electrodeof transistor 90 biases that transistor conductive to discharge voltageaccumulated on the capacitor 92. Accordingly, each pulse on conductor 74resets the capacitor 92 and initiates a new charging cycle for thecapacitor.

If the voltage accumulated on the capacitor attains a predeterminedlevel intermediate any pair of pulses on conductor 74, it will cause aunijunction transistor 104 to conduct. The unijunction transistor 104together with the resistors 106 and 108, the capacitor 92, and theconstant current source including transistor 94 comprises a conventionalrelaxation oscillator providing periodic output pulses across theresistor 108. The output pulses across resistor 108 occur when theemitter-base-one circuit of the unijunction transistor 104 conducts inresponse to voltage on capacitor 92.

A third transistor 110 included in the oscillator circuitry is connectedsuch that its base and emitter electrodes are connected across theresistor 108. A resistor 112 connects the collector of transistor 110with the battery 102. Each pulse from the relaxation oscillatordeveloped across resistor 108 biases transistor 110 conductive reducingthe voltage at the collector of the transistor 110 substantially toground potential. On the termination of conduction by unijunctiontransistor 104 and the concurrent termination of the voltage pulseacross resistor 108, the voltage level at the collector of transistor110 resumes a high level value related to the voltage of the battery102.

In response to the rising edge of the voltage at the collector oftransistor 110, a monostable multivibrator 114 provides an output pulseof a predetermined pulse width. It is noted that monostablemultivibrator 114 is of conventional design and units adaptable to thecircuit application are generally known and commercially available. Themonostable multivibrator 114 provides a high value output pulse of acontrolled pulse width in response to the rising edge of each inputpulse connected with the input to the monostable multivibrator.

An emitter follower output stage comprising a transistor 116 and aresistor 118 provides an output from the oscillator 78. It should beappreciated that an output pulse train is provided which conforms to thepulse output from the monostable multivibrator 114 developed in themanner described above. I

A conductor 82 connects the pulse output from the emitter follower withthe summer 80 where it is combined with the pulse train from the adder70 through resistor 119 on conductor 74, and the resultant pulsesummation is available on the conductor 84 at the input to the sawtoothgenerator 86. It is noted that a direct connection is shown foreffecting the summation of the pulse trains on conductors 74 and 82; inthe alternative, diode connections could be used to accomplish thesummation and maintain isolation between the two circuits.

The sawtooth generator 86 is reset by each pulse on the conductor 84.Conductor 84 is connected with the base electrode of a transistor 120;the transistor 120 having its collector and emitter electrodes connectedacross a capacitor 122. A resistor 124 connects the base and emitterelectrodes of the transistor 120. A current source comprising thetransistor 126, a source of direct voltage shown as a battery 128, andresistors 130 and 132 supplies charging current to the capacitor 122.The charging current has a level controllable in accordance with theoutput from the integrator connected with the base electrode anddescribed below. An emitter follower output stage comprising two directvoltage bias sources shown as batteries 134 and 136, a resistor 138 anda transistor 140 provides an output signal from the sawtooth generatoron conductor 142. This output signal has a sawtooth waveformcharacterized by a substantially constant maximum voltage independent ofthe frequency of sawtooth oscillations.

In operation, the current source connected with capacitor 122 providescharging current to develop a ramp voltage across the capacitor 122which is available at the output on conductor 142. Each pulse onconductor 84 resets the capacitor 122 since each pulse biases transistor120 conductive to discharge capacitor 122. In this manner, a sawtoothwaveform is developed at the output of the generator 86 on conductor 142having a frequency determined by the frequency of pulses on conductor84. At low frequencies, the frequency is substantially that of theconstant frequency oscillator 78; whereas at high frequencies when thepulses on conductor 84 are identically those on conductor 74, thefrequency of sawtooth oscillations is the same as the frequency ofpulses from the adder 70. During high frequency operation, where thefrequency of sawtooth oscillations varies according to changes in thefrequency of pulses on conductor 74, it is necessary to adjust thecurrent level provided by the current source to ensure that the maximumvoltage across capacitor 122 remains substantially constant.

To regulate the current level from the current source charging capacitor122, an operational amplifier 144, connected in an integrator circuit,supplies a control signal to the base electrode of transistor 126 whichin turn regulates the current level of the current source. A capacitor146 and positive and negative bias provided to the operational amplifier144 complete the necessary circuit connections to operate theoperational amplifier as an integrator. The operational amplifier 144 isof a conventional design generally known and commercially available. Theamplifier 144 has two inputs and its output is continuously adjusted byintegrating the difference between the respective inputs. When the twoinputs are equal, the output is maintained constant.

A control signal related to the desired average output from the sawtoothgenerator is developed by the battery 154 and the resistor 156 having avariable tap point for connection through resistor 158 with a firstinput to the operational amplifier 144. The sawtooth waveform developedat the output of the emitter follower across resistor 138 is filtered bya lowpass filter comprising resistors 160 and 162 and capacitors 164 and166. This filtered measure of the sawtooth generators output isconnected with the second input to the operational amplifier 144.

It should be appreciated that the filtered DC indication of the sawtoothgenerators output across resistor 138 is an indication of the maximumvoltage level attained by the sawtooth waveform during each cycle.Accordingly, the adjustment in current level from the current sourcecaused by the output from operational amplifier 144 will control themaximum voltage level and maintain it substantially constant throughoutthe operating range.

The sawtooth waveform on conductor 142 is connected with a voltagecomparator 168. In addition, a reference source of direct voltage shownas a battery is also connected to the voltage comparator 168. It isnoted that the battery 170 provides open loop control of the duty cycleof the voltage comparator 168, whereas the block diagram of FIG. 3 showsa control scheme providing feedback control. As discussed below, thechoice of control in a particular system would depend on systemrequirements. The comparator 168 is of conventional design, and itshould be appreciated that units of the type required are generallyknown and commercially available. The operation of voltage comparator168 is characterized by: a low level output when the two inputs have afirst voltage relation, for example, a low level output could beprovided when the sawtooth waveform on conductor 142 is less than thevoltage from the battery 170; a high level output when the two inputshave a second voltage relation, for example, a high level output couldbe provided when the sawtooth waveform on conductor 142 is greater thanthe voltage from the battery 170. This output from the voltagecomparator 168 is carried on a conductor 172. The remaining controlfeatures of the present invention will be described in conjunction withthe block diagram of FIG. 3.

The output sawtooth waveform from the sawtooth generator 86 is depictedin the graphs of FIGS. 4A, 4B, and 4C and is there denoted 142 inagreement with the notation of FIGS. 3 and 5 where conductor 142 carriesthe output sawtooth waveform. It is noted that the sawtooth waveform issynchronized with the trigger signals 74 in a manner such that a newcycle of the sawtooth is initiated at the outset of each 60 incrementduring both low frequency and high frequency operation. At frequenciesabove the frequency of the pulse oscillator 78 (high frequencyoperation), a single sawtooth cycle is completed during each 60 ofoperation. On the other hand, at frequencies below the frequency of thepulse oscillator 78 (low frequency operation), the sawtooth generatorcompletes more than one cycle in each 60 of operation. Thus, it shouldbe appreciated that the oscillator 78 and sawtooth generator 86comminute or fractionate the 60 intervals during low frequencyoperation. An integral number of sawtooth cycles or an integral numberplus a fractional cycle may be completed for each 60 at low frequenciesdepending on the relationship of the time of 60 and the time of onesawtooth cycle. All this follows directly from the description of thesawtooth generato' 86 set out above. It should be appreciated that theupper limit of low frequency operation can be set by adjusting thefrequency of the oscillator 78. During the time in which the oscillator78 provides output pulses, the frequency of the generator 86 issubstantially the same as the frequency of the oscillator 78 as shown inthe graphs of FIGS. 4A and 48 wherein one cycle of the sawtooth waveformis completed intermediate each pair of pulses from the oscillator 78. Asshown in FIG. 4C, when the frequency of pulses from the adder 70 exceedsthe fixed frequency of oscillator 78, the sawtooth waveform from thegenerator 86 has the same frequency as the pulses on conductor 74.

It is noted that the feedback control of sawtooth amplitude set outabove in conjunction with FIG. 5 is merely one scheme of regulating thesawtooth voltage during high frequency operation. In the alternative,the requisite voltage adjustment could be readily accomplished byregulation wherein an increasing current, for charging capacitor 122 inFIG. 5, is developed by connections with a tachometer speed sensingdevice. When a tachometer is used, the current is maintained constantuntil the speed of operation exceeds a predetermined level. Above thepreselected speed, the current is increased proportionately with thespeed to maintain a constant voltage sawtooth waveform.

Reference should now be made to FIG. 3 which is used to complete thedescription of the control afforded by the present invention. Thesawtooth signal on conductor 142 is applied to a voltage comparator 168for comparison with a DC reference or control voltage. As noted above,the comarator 168 provides a two state output on the conductor 172. Thetwo inputs to the comparator are connected with conductors 142 and 174in a manner such that the output from the comparator 168 is at its lowlevel when the sawtooth voltage has an instantaneous level less than thelevel of the reference DC voltage connected to the comparator and theoutput is at its high level when the sawtooth voltage has aninstantaneous level higher than the level of the reference DC voltage.This output on conductor 172 is depicted in the three graphs of FIGS.4A, 4B, and 4C and is there denoted 172.

In FIG. 3, a connection is made from conductor 172 to a low pass filter176 which senses the output from the voltage comparator 168. The filteraverages the output on conductor 172 and develops a DC indicationthereof. In operation, it should be appreciated that the level of the DCis an indication of the relationship between the time in whichcomparator 168 provides its high level output and the time during whichit provides its low level output.

This DC signal from the low-pass filter 176 which is proportional to theduty cycle of the comparator 168 is applied to one input of anintegrator 178. The integrator 178 has a second input connected with asource of direct voltage 180. The source of direct voltage 180 can takea variety of known forms including a regulated full wave rectified DCsource or simply a battery. The integrator 178 is of conventional designgenerally known in the art and is neither illustrated nor discussed indetail. One circuit suitable for inclusion in the circuit to perform thefunction of this integrator comprises a differential amplifier connectedto perform the requisite integration. In operation, the output of theamplifier is an integration of the difference between the input signalsfrom the source of direct voltage 180 and the low-pass filter 178.Accordingly, the system, by means of this feedback loop, makescontinuous adjustments until the DC level from the filter 176 is thesame as the DC level from the source of direct voltage 180.

As an alternative to the feedback control shown in FIG. 3, a directconnection from a reference DC voltage source could be made to thevoltage comparator 168. This direct connection (open loop) means ofcontrol is shown in FIG. 5 and was discussed above in conjunction withthat figure. The battery 170 of FIG. 5 provides the open loop control,and it should be noted by comparison that the feedback loop of FIG. 3comprising the filter 176 and the integrator 178 is eliminated by usingopen loop control. Performance requirements during operation woulddetermine the preferred circuit arrangement.

In either control scheme, the DC source (source of direct voltage 180 inFIG. 3 or battery 170 in FIG. 5) connected in the control affordsautomatic or operator control of the duty cycle of the voltagecomparator 168. By adjusting the level of the DC source, the duty cycleis modified, and the voltage the converter supplies the motor isregulated as more fully described below. It is noted that manual orautomatic control of the DC level can be provided according to thesystem requirements.

Continuing the description of the drawing of FIG. 3, the adder 70 isalso connected directly with a logic network 182 to control the timingof the sequential and periodic application of trigger signals to thevarious full wave bridge networks of FIG. 1. In FIG. 3, the A+ bridgetrigger for controlled rectifiers 36A and 38A is the only trigger outputshown. In accordance with the summary of the triggering system set forthabove, it should be appreciated that six connections are required forthe gate-cathode circuits of the controlled rectifiers 36A and 38A tocontrol their gating. It is noted that the logic 182 is connected withsimilar trigger control circuitry (not illustrated) for each of theremaining five bridge rectifier networks. This triggering system isdisclosed and discussed in detail, as noted above, in copendingapplication, Ser. No. 57,143.

A three-input AND gate 84 has two inputs connected with the logic 182 byconductors 186 and 188 and its third input connected with the conductor172. In this manner, the AND gate is controlled according to the threesignals connected with its inputs. The AND gate 184 is of conventionaldesign generally known in the art and commercially available. Itsoperation is such that it provides a low output level signal until andunless all three inputs connected with it are at a high level. When thethree inputs connected with the AND gate 184 are simultaneously at theirhigh level, the AND gate provides a high level output and the transistor190 is biased conductive. It should be appreciated that the logic 182disables the AND gate except during the intervals shown in the graph ofFIG. 2 and there denoted A+. Additionally, the third input to the ANDgate from the conductor 172 effects pulse modulation during the intervalA+ of the graph of FIG. 2. Accordingly, transistor 190 is cycledconductive and nonconductive depending on the output level availablefrom the AND gate 184. This pulse modulation is shown in the graphs ofFIGS. 4A, 4B, and 4C.

It should be understood that the description directed to the developmentof trigger signal A+ is analogous to that which applies to the remainingfive trigger outputs (not illustrated) required for the FIG. 1 converterarrangement. The generation of trigger signal A+ is accomplishedaccording to the summary set out above and by way of reiteration, it isnoted that the square wave oscillator or chopper 192 connected withtransistor 190 energizes the primary winding of a transformer which inturn excites six secondary windings to develop six signals forcontrolled rectifiers 36A and 38A. The trigger circuitry 194 encompassesthe transformers and rectifier circuitry required as well as six triggercircuits. Again, this is duplicated five times to accommodate theremaining five full wave bridge rectifiers. Of course, the timing of theremaining trigger circuits is different from that shown for the A+triggers. Essentially, the remaining controlled rectifiers are suppliedgate turn-on pulses during the respective times depicted for the onecycle of operation in the graph of FIG. 2 with modulation similar tothat shown for the A+ triggers in FIGS. 4A, 4B, and 4C.

Reference should now be made to FIGS. 4A, 4B, and 4C wherein the effectof the duty cycle modulation of this invention is graphically summarizedfor the A+ trigger outputs. It should be appreciated, as noted above,that the A+ output shown in the graphs is applied to all six controlledrectifiers 36A and 38A. The remaining five bridge networks are modulatedin an analogous fashion during their respective conductive intervals. Inthe three figures, the frequency of pulse train 74 has a varying ratioto the pulse train frequency for signal 82. FIG. 4A depicts thesituation at low motor frequency and the A+ trigger is shown for asingle interval. FIG. 4B shows the operation at an intermediatefrequency, and FIG. 4C summarizes the operation at high frequency.

As shown in FIG. 4A, the sawtooth waveform 142 begins a new cycle on theoccurrence of each pulse of the pulse train 84. The DC reference V (174)from the integrator 178 of FIG. 3 is shown superposed on the sawtoothwaveform 142, and the output waveform 172 from the voltage comparator168 is at its high state when the sawtooth voltage 142 exceeds the DCreference V and is at its low state when the sawtooth 142 is less thanthe DC reference V Trigger signals for the A+ bridge rectifier areconstrained to occur in the zero to 120 time interval as a consequenceof the control provided by the logic 182 to AND gate 184 with theadditional constraint imposed that A+ triggers are present in thisinterval only when waveform 172 is at its high state. With these factorsin mind, the A+ trigger depicted in FIG. 4A follows.

FIG. 4B shows a second operating frequency somewhat higher than that ofFIG. 4A. Accordingly, the frequency of the fixed frequency oscillator 78generating pulse train 82 is closer to the frequency from the adder fromline 74. Each pulse in the pulse train 84 rests the sawtooth generator86, and accordingly, the sawtooth waveform 142 has the waveform shown inthe figure. It is noted that the sawtooth waveform 142 completesslightly more than one cycle during each 60 time increment. The waveform172 is at its high level when the sawtooth 142 has an instantaneousvoltage in excess of the reference voltage V As was the case for FIG.4A, the A+ trigger is present only during those time intervals from zeroto 120 with the additional constraint that it is inhibited when thewaveform 142 is at its low level.

In FIG. 4C, the frequency of adder 70 exceeds the frequency of the fixedfrequency oscillator 78, and the pulse train 82 is eliminated.Accordingly, as noted above, pulse train 84 is identical to pulse train74, and the sawtooth waveform 142 has the same frequency as pulse train74 completing a single cycle in each 60. The waveform 172 has a highlevel when the sawtooth voltage of waveform 142 exceeds the reference Vand the A+ trigger comprises a single pulse for each 60 of motoroperation as shown in the figure.

Waveform 172 is connected with the trigger logic for each of the sixbridge rectifier networks. Accordingly, each is constrained by the pulsemodulation control such that it is conductive only when the waveform 172is at its high level. In addition, each of the bridge rectifier networksis controlled by the logic 182 as described above.

It should be understood from the foregoing that the voltage supplied themotor by the converter is controlled by regulating the voltage VD shownin the graphs superposed on the sawtooth waveform 142. For example, ifthe voltage from the source of direct voltage 180 of FIG. 3 isincreased, the A+ trigger shown in FIGS. 4A, 4B, and 4C will reduce thelength of trigger pulses applied to controlled rectifiers 36A and 38A toreduce the voltage supplied the motor. Similarly, if the voltage fromthe source of direct voltage 180 is decreased, the voltage supplied themotor willincrease.

The present invention affords two modes of pulse modulation to controlthe power level supplied to the AC induction motor. The first mode,associated with low frequency operation, uses an auxiliary oscillator toincrease the modulation frequency above that associated with normal 60modulation. The second or high frequency mode of operation reliesexclusively on 60 modulation to regulate the voltage level. Triggersignals to various bridge rectifiers are inhibited during each cycle ofmodulation and smooth transition is effected as the number of cycles ofmodulation decreases continuously with increasing motor frequency untilthe high frequency mode is reached.

Although this invention has been described in terms of a preferredembodiment, it should be appreciated that various changes andmodifications could be engrafted upon the example by one skilled in theart within the scope of the claims appended hereto. Particularly, itshould be borne in mind that inverters providing an AC output from a DCinput as well as cycloconverters providing an AC output from an AC inputcan use the regulation scheme of this invention including voltagecontrol conduction periods and frequency control conduction periods asdescribed above. Additionally, it should be understood that thecontrolled switches of the converter can be controlled rectifiers,transistors or other controllable switching devices.

We claim:

1. A power supply system for supplying a variable frequency alternatingvoltage having a variable voltage magnitude to an electrical load,comprising: a source of power, converter means including a plurality ofcontrollable switching devices connected between said source of powerand said electrical load for controlling both the magnitude andfrequency of the alternating voltage applied to said electrical loadfrom said converter means, variable frequency control means coupled tosaid switching devices for causing predetermined switching devices to beconductive during predetermined frequency determining conduction periodsto thereby determine the frequency of the alternating voltage applied tosaid electrical load, voltage control means coupled to said switchingdevices for causing said predetermined switching devices to beconductive and nonconductive during a given frequency determiningconduction period to define a plurality of voltage control conductionperiods within said given frequency determining conduction period tocontrol the magnitude of alternating voltage applied to said load, aconstant frequency pulse generator and means for initiating andterminating the operation of said pulse generator as a function of theoutput of said variable frequency control means whereby, said pulsegenerator initiates a cycle at the outset of each frequency determiningconduction period and produces a number of pulses determined by theintegral number of cycles completed by said pulse generator during thefrequency determining conduction period and whereby, said pulsegenerator produces no output pulses when the frequency of said frequencycontrol means exceeds the frequency of said pulse generator, meanscoupling the output of said pulse generator and the output of saidfrequency control means to said voltage control means to determine thenumber of voltage control conduction periods whereby, the number ofvoltage control conduction periods when the frequency of said variablefrequency control means is less than the frequency of said pulsegenerator isequal to or one greater than the number of pulses providedby said pulse generator during a given frequency determining conductionperiod and whereby, a single voltage control conduction period isprovided within a given frequency determining conduction period when thefrequency of said variable frequency control means is greater than thefrequency of said pulse generator, and means for controlling thepercentage on-time for said switching devices during each of saidvoltage control conduction periods.

2. A power supply system for supplying a variable frequency alternatingvoltage having a variable voltage magnitude to an alternating currentmotor having a winding, comprising: a source of power, converter meansincluding a plurality of controllable switching devices connectedbetween said source of power and said motor winding for controlling thefrequency and voltage magnitude of the alternating voltage applied tosaid motor winding from said converter means, variable frequency controlmeans coupled to said switching devices for causing predeterminedswitching devices to be conductive during predetermined frequencydetermining conduction periods to thereby determine the frequency of thealternating voltage applied to said motor winding, voltage control meanscoupled to said switching devices for causing said predeterminedswitching devices to be conductive and nonconductive during a givenfrequency determining conduction period to thereby define a plurality ofvoltage control conduction periods within a frequency determiningconduction period to control the magnitude of alternating voltageapplied to said motor winding, a constant frequency pulse generator,means for initiating and terminating the operation of said pulsegenerator as a function of the frequency of said variable frequencycontrol means whereby, said pulse generator produces a predeterminednumber of pulses during a given frequency determining conduction periodand'produces no output pulses when the frequency of said variablefrequency control means exceeds the operating frequency of said pulsegenerator, means coupling the output of said pulse generator and theoutput of said variable frequency control means to said voltage controlmeans whereby, the number of voltage control conduction periods withineach frequency determining conduction period is a function of theoperating frequency of said pulse generator when the frequency of saidpulse generator is greater than the frequency of said variable frequencycontrol means and is a function of the operating frequency of saidvariable frequency control means when the frequency of said pulsegenerator is less than the frequency of said variable frequency controlmeans, and means for controlling the pulse width of voltage pulsessupplied said motor winding during said voltage control conductionperiods.

3. A power supply system for an alternating current motor having awinding, comprising: a source of voltage, converter means including aplurality of controllable switching devices connected between saidsource of voltage and said motor winding, means coupled to saidconverter means for controlling the switching frequency of saidswitching devices such that an alternating current of a predeterminedcontrollable frequency is applied to said motor winding from saidconverter means, a variable frequency ramp voltage generating means fordeveloping a series of ramp voltages at a controllable frequency, asource of direct controllable voltage, comparator means coupled to saidswitching devices, to said source of direct control voltage and to saidramp voltage generating means for controlling the conduction periods ofsaid switching devices to thereby control the voltage applied to saidmotor winding, and control means controlled as a function of theswitching frequency of said converter and coupled to said ramp voltagegenerating means for causing said ramp voltage generating means tooperate at a higher frequency than the switching frequency of saidconverter when said switching frequency is below a predetermined valueand for operating said ramp voltage generating means substantially atthe switching frequency of said converter means when said switchingfrequency is above a predetermined value.

4. An AC motor control system, comprising: a power supply, a controlledrectifier converter, an AC motor, means connecting said power supply tothe input of said converter, means connecting said AC motor to theoutput of said converter, a source of control signals connected with thecontrolled rectifiers of said converter to control the frequency ofpower supplied said AC motor, said source of control signals generatingswitching signals at a controllable predetermined frequency to affordcontinuous control of the frequency of power supplied said AC motor, aconstant frequency oscillator, a constant amplitude variable frequencysawtooth wave generator, means coupling said source of control signalsand said constant frequency oscillator with said sawtooth generator,said oscillator and said source of control signals being so connectedwith said sawtooth generator that said sawtooth generator operates atthe frequency of said switching signals when the frequency of saidswitching signals exceeds the frequency of said constant frequencyoscillator and operates substantially at the frequency of said constantfrequency oscillator when the frequency of said switching signals isless than the frequency of said constant frequency oscillator, avariable amplitude DC reference source, means connected with both saidsawtooth generator and said DC reference source for comparing thevoltage magnitude of said DC reference source with the instantaneousvoltage magnitude from said sawtooth generator, and a control meansconnected with the controlled rectifiers of said converter and with saidcomparing means to inhibit conduction by said controlled rectifiers whenthe DC reference voltage has a first predetermined relation to thevoltage of said sawtooth generator and to permit conduction by saidcontrolled rectifiers when the DC reference voltage has a secondpredetermined relation to the voltage of said sawtooth generatorwhereby, a variable duty cycle is effected to regulate the power levelavailable to said AC motor.

5. An AC motor control system, comprising: an alternating voltage powersupply, a controlled rectifier converter, an AC motor, means connectingsaid alternating voltage power supply to the input of said converter,means connecting said AC motor to the output of said converter, 21source of gating signals connected with the controlled rectifiers ofsaid converter to control the frequency of power supplied said AC motor,a control means included in said source of gating signals to effectcontrol of gating signals to the respective controlled rectifiers, saidsource of gating signals generating switching signals at a controllablepredetermined frequency to afford continuous control of the frequency ofpower supplied said AC motor, a constant frequency pulse generator, aconstant amplitude variable frequency sawtooth wave generator, meanscoupling said source of gating signals and said constant frequency pulsegenerator with said sawtooth generator, said sawtooth generatoroperating at the frequency of said switching signals when the frequencyof said switching signals exceeds the frequency of said constantfrequency pulse generator, said sawtooth generator operatingsubstantially at the frequency of said constant frequency pulsegenerator when the frequency of said switching signals is less than thefrequency of said constant frequency pulse generator, a variableamplitude DC reference source, means connected with both said sawtoothgenerator and said DC reference source for comparing the voltagemagnitude of said DC reference source with the instantaneous magnitudefrom said sawtooth generator, and means connecting said control meanswith said comparing means to inhibit or enable the application ofswitching signals by said source of gating signals to precludeconduction by the controlled rectifiers of said converter when the DCreference voltage has a magnitude larger than the instantaneousmagnitude of the voltage of said sawtooth generator and to permitconduction by the controlled rectifiers of said converter when the DCreference voltage has a magnitude less than the instantaneous magnitudeof the voltage of said sawtooth generator whereby, a variable duty cycleis effected to regulate the power level available to said AC motor.

6. The motor control system according to claim 5 where the source ofgating signals gates the controlled rectifiers such that substantiallyrectangular voltage pulses are applied to said motor from saidconverter.

7. A motor control system for a polyphase induction motor having apolyphase winding and a squirrel cage rotor, comprising: a source ofpower, a switching circuit comprising a plurality of controllableswitching devices connected between said source of power and saidwinding for controlling both the magnitude and frequency of thealternating voltage applied to said motor from said switching circuit,variable frequency control means coupled to said switching devices forcausing predetermined switching devices to be conductive duringpredetermined frequency determining conduction periods to therebydetermine the frequency of the alternating voltage applied to said motorwinding, voltage control means coupled to said switching devices forcausing said predetermined switching devices to be conductive andnonconductive during a given frequency determining conduction period todefine a plurality of voltage control conduction periods within saidgiven frequency determining conduction period to control the magnitudeof alternating voltage applied to said motor winding, said voltagecontrol means including means for determining the number of voltagecontrol conduction periods that occur during a given frequencydetermining conduction period and for decreasing the number of voltagecontrol conduction periods that occur in a given frequency determiningconduction period as the switching frequency of said switching circuitincreases when the switching frequency of said switching circuit isbelow a predetermined value, said voltage control means including meansfor causing one voltage control conduction period to occur during eachfrequency determining conduction period when the switching frequency ofsaid switching circuit is above said predetermined value whereby, asmooth continuous transition occurs in the number of voltage controlconduction periods as the switching frequency of said switching circuitvaries, and variable control means for controlling the pulse width ofvoltage pulses supplied said winding during said voltage controlconduction periods whereby, the average voltage applied to said motorwinding can be controlled to thereby provide a system wherein both theaverage voltage and frequency of the alternating voltage applied to saidinduction motor is controllable.

8. The motor control system according to claim 7 where the variablefrequency control means includes means for controlling the slipfrequency of said induction motor.

9. A vehicle propulsion system, comprising: an AC induction motor havinga winding and a squirrel cage rotor, means coupling said rotor to avehicle wheel, an alternating current generator, a prime movermechanically coupled to said generator for driving said generator, acontrolled rectifier converter connected between said generator and saidmotor winding for controlling both the magnitude and frequency of thealternating voltage supplied to said motor winding from said converter,a source of control signals connected with the controlled rectifiers ofsaid converter to control the frequency of power supplied said motorwinding, said source of control signals generating switching signals ata controllable predetermined frequency to afford continuous control ofthe frequency of power supplied said AC motor, a constant frequencyoscillator, a constant amplitude variable frequency sawtooth wavegenerator, means coupling said source of control signals and saidconstant frequency oscillator with said sawtooth generator, saidoscillator and said source of control signals being so connected withsaid sawtooth generator that said sawtooth generator operates at thefrequency of said switching signals when the frequency of said switchingsignals exceeds the frequency of said constant frequency oscillator andoperates substantially at the frequency of said constant frequencyoscillator when the frequency of said switching means is less than thefrequency of said constant frequency oscillator, a variable amplitude DCreference source, means connected with both said sawtooth generator andsaid DC reference source for comparing the voltage magnitude of said DCreference source with the instantaneous voltage magnitude from saidsawtooth generator, and a control means connected with the controlledrectifiers of said converter and with said comparing means to inhibitconduction by said controlled rectifrers when the DC reference voltagehas a first predetermined relation to the voltage of said sawtoothgenerator and to permit conduction by said controlled rectifiers whenthe DC reference voltage has a second predetermined relation to thevoltage of said sawtooth generator whereby, a variable duty cycle iseffected to regulate the power level available to said AC motor.

* i =IK

1. A power supply system for supplying a variable frequency alternatingvoltage having a variable voltage magnitude to an electrical load,comprising: a source of power, converter means including a plurality ofcontrollable switching devices connected between said source of powerand said electrical load for controlling both the magnitude andfrequency of the alternating voltage applied to said electrical loadfrom said converter means, variable frequency control means coupled tosaid switching devices for causing predetermined switching devices to beconductive during predetermined frequency determining conduction periodsto thereby determine the frequency of the alternating voltage applied tosaid electrical load, voltage control means coupled to said switchingdevices for causing said predetermined switching devices to beconductive and nonconductive during a given frequency determiningconduction period to define a plurality of voltage control conductionperiods within said given frequency determining conduction period tocontrol the magnitude of alternating voltage applied to said load, aconstant frequency pulse generator and means for initiating andterminating the operation of said pulse generator as a function of theoutput of said variable frequency control means whereby, said pulsegenerator initiates a cycle at the outset of each frequency determiningconduction period and produces a number of pulses determined by theintegral number of cycles completed by said pulse generator during thefrequency determining conduction period and whereby, said pulsegenerator produces no output pulses when the frequency of said frequencycontrol means exceeds the frequency of said pulse generator, meanscoupling the output of said pulse generator and the output of saidfrequency control means to said voltage control means to determine thenumber of voltage control conduction periods whereby, the number ofvoltage control conduction periods when the frequency of said variablefrequency control means is less than the frequency of said pulsegenerator is equal to or one greater than the number of pulses providedby said pulse generator during a given frequency determining conductionperiod and whereby, a single voltage control conduction period isprovided within a given frequency determining conduction period when thefrequency of said variable frequency control means is greater than thefrequency of said pulse generator, and means for controlling thepercentage on-time for said switching devices during each of saidvoltage control conduction periods.
 2. A power supply system forsupplying a variable frequency alternating voltage having a variablevoltage magnitude to an alternating current motor having a winding,comprising: a source of power, converter means including a plurality ofcontrollable switching devices connected between said source of powerand said motor winding for controlling the frequency and voltagemagnitude of the alternating voltage applied to said motor winding fromsaid converter means, variable frequency control means coupled to saidswitching devices for causing predetermined switching devices to beconductive during predetermined frequency determining conduction periodsto thereby determine the frequency of the alternating voltage applied tosaid motor winding, voltage control means coupled to said switchingdevices for causing said predetermined switching devices to beconductive and nonconductive during a given frequency determiningconduction period to thereby define a plurality of voltage controlconduction periods within a frequency determining conduction period tocontrol the magnitude of alternating voltage applied to said motorwinding, a constant frequency pulse generator, means for initiating andterminating the operation of said pulse generator as a function of thefrequency of said variable frequency control means whereby, said pulsegenerator produces a predetermined number of pulses during a givenfrequency determining conduction period and produces no output pulseswhen the frequency of said variable frequency control means exceeds theoperating frequency of said pulse generator, means coupling the outputof said pulse generator and the output of said variable frequencycontrol means to said voltage control means whereby, the number ofvoltage control conduction periods within each frequency determiningconduction period is a function of the operating frequency of said pulsegenerator when the frequency of said pulse generator is greater than thefrequency of said variable frequency control means and is a function ofthe operating frequency of said variable frequency control means whenthe frequency of said pulse generator is less than the frequency of saidvariable frequency control means, and means for controlling the pulsewidth of voltage pulses supplied said motor winding during said voltagecontrol conduction periods.
 3. A power supply system for an alternatingcurrent motor having a winding, comprising: a source of voltage,converter means including a plurality of controllable switching devicesconnected between said source of voltage and said motor winding, meanscoupled to said converter means for controlling the switching frequencyof said switching devices such that an alternating current of apredetermined controllable frequency is applied to said motor windingfrom said converter means, a variable frequency ramp voltage generatingmeans for developing a series of ramp voltages at a controllablefrequency, a source of direct controllable voltage, comparator meanscoupled to said switching devices, to said source of direct controlvoltage and to said ramp voltage generating means for controlling theconduction periods of said switching devices to thereby control thevoltage applied to said motor winding, and control means controlled as afunction of the switching frequency of said converter and coupled tosaid ramp voltage generating means for causing said ramp voltagegenerating means to operate at a higher frequency than the switchingfrequency of said converter when said switching frequency is below apredetermined value and for operating said ramp voltage generating meanssubstantially at the switching frequency of said converter means whensaid switching frequency is above a predetermined value.
 4. An AC motorcontrol system, comprising: a power supply, a controlled rectifierconverter, an AC motor, means connecting said power supply to the inputof said converter, means connecting said AC motor to the output of saidconverter, a source of control signals connected with the controlledrectifiers of said converter to control the frequency of power suppliedsaid AC motor, said source of control signals generating switchingsignals at a controllable predetermined frequency to afford continuouscontrol of the frequency of power supplied said AC motor, a constantfrequency oscillator, a constant amplitude variable frequency sawtoothwave generator, means coupling said source of control signals and saidconstant frequency oscillator with said sawtooth generator, saidoscillator and said source of control signals being so connected withsaid sawtooth generator that said sawtooth generator operates at thefrequency of said switching signals when the frequency of said switchingsignals exceeds the frequency of said constant frequency oscillator andoperates substantially at the frequency of said constant frequencyoscillator when the frequency of said switching signals is less than thefrequency of said constant frequency oscillator, a variable amplitude DCreference source, means connected with both said sawtooth generator andsaid DC reference source for comparing the voltage magnitude of said DCreference source with the instantaneous voltage magnitude from saidsawtooth generator, and a control means connected with the controlledrectifiers of said converter and with said comparing means to inhibitconduction by said controlled rectifiers when the DC reference voltagehas a first predetermined relation to the voltage of said sawtoothgenerator and to permit conduction by said controlled rectifiers whenthe DC reference voltage has a second predetermined relation to thevoltage of said sawtooth generator whereby, a variable duty cycle iseffected to regulate the power level available to said AC motor.
 5. AnAC motor control system, comprising: an alternating voltage powersupply, a controlled rectifier converter, an AC motor, means connectingsaid alternating voltage power supply to the input of said converter,means connecting said AC motor to the output of said converter, a sourceof gating signals connected with the controlled rectifiers of saidconverter to control the frequency of power supplied said AC motor, acontrol means included in said source of gating signals to effectcontrol of gating signals to the respective controlled rectifiers, saidsource of gating signals generating switching signals at a controllablepredetermined frequency to afford continuous control of the frequency ofpower supplied said AC motor, a constant frequency pulse generator, aconstant amplitude variable frequency sawtooth wave generator, meanscoupling said source of gating signals and said constant frequency pulsegenerator with said sawtooth generator, said sawtooth generatoroperating at the frequency of said switching signals when the frequencyof said switching signals exceeds the frequency of said constantfrequency pulse generator, said sawtooth generator operatingsubstantially at the frequency of said constant frequency pulsegenerator when the frequency of said switching signals is less than thefrequency of said constant frequency pulse generator, a variableamplitude DC reference source, means connected with both said sawtoothgenerator and said DC reference source for comparing the voltagemagnitude of said DC reference source with the instantaneous magnitudefrom said sawtooth generator, and means connecting said control meanswith said comparing means to inhibit or enable the application ofswitching signals by said source of gating signals to precludeconduction by the controlled rectifiers of said converter when the DCreference voltage has a magnitude larger than the instantaneousmagnitude of the voltage of said sawtooth generator and to permitconduction by the controlled rectifiers of said converter when the DCreference voltage has a magnitude less than the instantaneous magnitudeof the voltage of said sawtooth generator whereby, a variable duty cycleis effected to regulate the power level available to said AC motor. 6.The motor control system according to claim 5 where the source of gatingsignals gates the controlled rectifiers such that substantiallyrectangular voltage pulses are applied to said motor from saidconverter.
 7. A motor control system for a polyphase induction motorhaving a polyphase winding and a squirrel cage rotor, comprising: asource of power, a switching circuit comprising a plurality ofcontrollable switching devices connected between said source of powerand said winding for controlling both the magnitude and frequency of thealternating voltage applied to said motor from said switching circuit,variable frequency control means coupled to said switching devices forcausing predetermined switching devices to be conductive duringpredetermined frequency determining conduction periods to therebydetermine the frequency of the alternating voltage applied to said motorwinding, voltage control means coupled to said switching devices forcausing said predetermined switching devices to be conductive andnonconductive during a given frequency determining conduction period todefine a plurality of vOltage control conduction periods within saidgiven frequency determining conduction period to control the magnitudeof alternating voltage applied to said motor winding, said voltagecontrol means including means for determining the number of voltagecontrol conduction periods that occur during a given frequencydetermining conduction period and for decreasing the number of voltagecontrol conduction periods that occur in a given frequency determiningconduction period as the switching frequency of said switching circuitincreases when the switching frequency of said switching circuit isbelow a predetermined value, said voltage control means including meansfor causing one voltage control conduction period to occur during eachfrequency determining conduction period when the switching frequency ofsaid switching circuit is above said predetermined value whereby, asmooth continuous transition occurs in the number of voltage controlconduction periods as the switching frequency of said switching circuitvaries, and variable control means for controlling the pulse width ofvoltage pulses supplied said winding during said voltage controlconduction periods whereby, the average voltage applied to said motorwinding can be controlled to thereby provide a system wherein both theaverage voltage and frequency of the alternating voltage applied to saidinduction motor is controllable.
 8. The motor control system accordingto claim 7 where the variable frequency control means includes means forcontrolling the slip frequency of said induction motor.
 9. A vehiclepropulsion system, comprising: an AC induction motor having a windingand a squirrel cage rotor, means coupling said rotor to a vehicle wheel,an alternating current generator, a prime mover mechanically coupled tosaid generator for driving said generator, a controlled rectifierconverter connected between said generator and said motor winding forcontrolling both the magnitude and frequency of the alternating voltagesupplied to said motor winding from said converter, a source of controlsignals connected with the controlled rectifiers of said converter tocontrol the frequency of power supplied said motor winding, said sourceof control signals generating switching signals at a controllablepredetermined frequency to afford continuous control of the frequency ofpower supplied said AC motor, a constant frequency oscillator, aconstant amplitude variable frequency sawtooth wave generator, meanscoupling said source of control signals and said constant frequencyoscillator with said sawtooth generator, said oscillator and said sourceof control signals being so connected with said sawtooth generator thatsaid sawtooth generator operates at the frequency of said switchingsignals when the frequency of said switching signals exceeds thefrequency of said constant frequency oscillator and operatessubstantially at the frequency of said constant frequency oscillatorwhen the frequency of said switching means is less than the frequency ofsaid constant frequency oscillator, a variable amplitude DC referencesource, means connected with both said sawtooth generator and said DCreference source for comparing the voltage magnitude of said DCreference source with the instantaneous voltage magnitude from saidsawtooth generator, and a control means connected with the controlledrectifiers of said converter and with said comparing means to inhibitconduction by said controlled rectifiers when the DC reference voltagehas a first predetermined relation to the voltage of said sawtoothgenerator and to permit conduction by said controlled rectifiers whenthe DC reference voltage has a second predetermined relation to thevoltage of said sawtooth generator whereby, a variable duty cycle iseffected to regulate the power level available to said AC motor.